Selectively permeable (also referred to as permselective) asymmetric membranes are particularly advantageous when used in connection with various types of medical treatments, such as hemodialysis, hemofiltration, plasmapheresis, and immunotherapy. Such membranes may also be used in connection with dialysis and filtration in general, such as in connection with cleaning or depyrogenization of water.
These types of membranes are described in detail, for example, in EP-A-O168 783, EP-B-O082 433, and WO 86/00028. The contents of these publications are therefore included in the present application by reference thereto. Referring specifically to the latter publication (WO 86/00028), there is disclosed therein asymmetrical hollow fiber membranes which include a relatively dense skin layer of less than about 1 micron on their outer periphery, bounded by an open structure of increasing porosity towards the inner surface of the hollow fibers, which porous structure comprises micro-voids having the shape of elongated fingers open to the inner surface of the membranes. This publication also discloses the methods of obtaining these asymmetric hollow fibers via known spinning techniques by a dry-wet process which includes adding to the spinning solution of the polymer in the solvent at least one specified additive and adding a particular fluid through the center. FIG. 7 in this publication shows this particular structure.
The other two cited publications also disclose asymmetric membranes of this type comprised of mixtures of hydrophobic and hydrophilic polymers.